Chapter 5 Membrane Structure & Function
Feb 25, 2016
Chapter 5Membrane Structure & Function
Membrane structure, ISelective permeabilityAmphipathic
polar & non-polar regions(ex. hydrophilic & hydrophobic)
Davson-Danielli (1935-1970)protein “sandwich”
Singer-Nicolson: “fluid mosaic” model
currently widely accepted
Membrane structure, IIPhospholipids membrane fluidityCholesterol fluidity/stabilization“Mosaic” StructureIntegral proteins
trans-membrane proteinsPeripheral proteins surface of
membraneMembrane carbohydrates
cell to cell recognition; Oligosaccharides (cell markers);
glycolipidsglycoproteins
Membrane structure, III
Membrane protein function:•transport•enzymatic activity•signal transduction•intercellular joining•cell-cell recognition•ECM attachment
Membrane trafficDiffusion movement of a substance from
an area of high concentration of that substance to an area of low concentration of that substance.
Concentration gradient imbalance in concentration
Passive transport diffusion of a substance across a biological membrane without the need for energy
Osmosis the diffusion of water from an are of high concentration of water to an are of low concentration of water across a selectively permeable membrane
High Concentration
Low Concentration
Passive (with gradient)
Diffusion, Osmosis, Facilitated Diffusion
Active-Need ATP (against (or with) gradient)
Active Transport
Water balanceOsmoregulation control of
water balanceHypertonic higher
concentration of solutesHypotonic lower
concentration of solutesIsotonic equal
concentrations of solutesCells with Walls:Turgid (very firm)Flaccid (limp)Plasmolysis plasma membrane pulls away
from cell wall
Specialized TransportTransport proteinsFacilitated diffusion
passage of molecules and ions with transport proteins across a membrane down the concentration gradient
Active transport movement of a substance against its concentration gradient with the help of cellular energy
Types of Active TransportSodium-potassium pumpExocytosis secretion of
macromolecules by the fusion of vesicles with the plasma membrane
Endocytosis import of macromolecules by forming new vesicles with the plasma membrane•phagocytosis•pinocytosis •receptor-mediated endocytosis (ligands)
A Closer Look at Cell Membranes
Aim: How do large particles enter and exit cells?
Do Now: Name some molecules/materials that enter and exit the cell.
How would you describe the cell membrane that allows passage of these materials?
Exocytosis and Endocytosis
Exocytosis (out of the cell)• The fusion of a vesicle with the cell membrane,
releasing its contents to the surroundings
Endocytosis (into the cell)• The formation of a vesicle from cell membrane,
enclosing materials near the cell surface and bringing them into the cell
Endocytosis and Exocytosis Examples
Endocytosis and Exocytosis Examples
Three Pathways of Endocytosis
Bulk-phase endocytosis• Extracellular fluid is captured in a vesicle and
brought into the cell; the reverse of exocytosis
Receptor-mediated endocytosis• Specific molecules bind to surface receptors,
which are then enclosed in an endocytic vesicle
Phagocytosis• Pseudopods engulf target particle and merge as
a vesicle, which fuses with a lysosome in the cell
Phagocytosis (“engulfment”)
Membrane Cycling
Exocytosis and endocytosis continually replace and withdraw patches of the plasma membrane
New membrane proteins and lipids are made in the ER, modified in Golgi bodies, and form vesicles that fuse with plasma membrane
Exocytic Vesicle
5.5 Key Concepts: Membrane Trafficking
Large packets of substances and engulfed cells move across the plasma membrane by processes of endocytosis and exocytosis
Membrane lipids and proteins move to and from the plasma membrane during these processes